1. Field of the Invention
The present invention relates generally to a mobile communication terminal which includes a camera having an automatic gain control (AGC) function, and in particular, to a method for judging environmental brightness and controlling display brightness.
2. Description of the Related Art
Recently, a mobile communication terminal which not only displays motion pictures but also transmits/receives images through a camera has been developed. The camera for the mobile communication terminal is an image pickup device such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), and has an AGC function. The AGC function automatically controls light, so that a luminance average of an image for an AGC reference region set up in the center region of a photographed screen can exist within a predetermined range. It is thus possible to sense a subject in a bright or dark environment. The AGC function will now be briefly explained. An average of the luminance values of image signals sensed by sensors installed in the center region corresponding to the AGC reference region among the sensors of the image pickup device is calculated and confirmed to exist in or exceed a predetermined critical range. Thereafter, an iris is controlled to increase or decrease light, so that the average of the luminance values can exist within the critical range. When the average of the luminance values is greater than an upper limit value of the critical range, light is decreased. When it is les than a lower limit value of the critical range, light is increased. By repeating the procedure, an optimal image of the subject can be obtained in a bright or dark place.
On the other hand, the above-described mobile communication terminal uses a liquid crystal display (LCD) as a display device. The display brightness of the LCD seems different to a user according to the environmental brightness. In the case of a portable terminal carried by a user, the environmental brightness varies considerably. Accordingly, in order for the user to clearly see the image displayed on the LCD screen, the display brightness of the LCD must be appropriately controlled according to the environmental brightness. For example, the display brightness should be increased in a dark place.
Exemplary techniques for controlling display brightness include Korea Patent Publication Nos. 1998-013821 ‘Apparatus For Automatically Controlling Brightness Of Liquid Crystal Display’ (May 15, 1998), 1999-000122 ‘Method For Automatically Controlling Brightness Of Screen Of Liquid Crystal Display’ (Jan. 15, 1999), and 2000-0032894 ‘Method For Automatically Controlling Brightness Of Monitor Of Image Telephone’ (Jun. 15, 2000). Here, Korea Patent Publication No. 1998-013821 suggests a method for sensing the environmental rightness by using a solar cell, and automatically controlling the brightness of a ear-side light and a tilt so as to control the brightness of the LCD of a view finder of a camera. In addition, Korea Patent Publication No. 1999-000122 discloses a method for sensing luminous intensity by using an open/close state of an iris in a camcorder having the LCD, and automatically controlling the rightness, contrast and colors of the LCD screen, so that users can clearly see a subject. Korea Patent Publication No. 2000-0032894 teaches a method for interpreting a luminance signal level of an image photographed by a camera in an image telephone without using a luminance sensor, determining the environmental brightness, obtaining the optimal LCD brightness according to the result, and variably controlling the brightness of an LCD monitor.
As described above, the display brightness of the LCD is automatically controlled according to the environmental brightness. However, Korea Patent Publication No. 1998-013821 does not relate to the display of the mobile communication terminal, and requires a solar cell and a related circuit for sensing the environmental brightness. Moreover, since the environmental brightness is sensed merely by the light sensed by the solar cell, the display brightness may be inappropriately controlled. For example, when the solar cell is installed in front of the LCD and a bright light source is located behind the LCD, the solar cell determines the environmental brightness as a bright state by sensing light reflected by the user. But the user sees the LCD screen relatively dark.
In addition, when it is presumed that a method for sensing the luminous intensity according to an open/close state of an iris, or determining the environmental brightness on the basis of a luminance signal level of an image photographed by a camera is applied to a mobile communication terminal including a camera having the AGC function, the AGC function fails to precisely determine the environmental brightness. That is, although a subject and a background often have different brightness in the mobile communication terminal environment, the AGC function controls the light to exist within the critical range only in the AGC reference region which is part of the whole screen, thereby relatively distorting the environmental brightness. For instance, in a backlight state, the environmental brightness is high but the subject doesn't look clearly. Therefore, the AGC function opens the iris more and increases light to brighten the subject. In this case, if the luminous intensity of the environment is sensed according to the open/close state of the iris, the environmental brightness is misdetermined as the dark state. In addition, when the environmental brightness is determined according to the luminance signal level of the whole screen of the image photographed by the camera, if a bright light source is positioned in front of a subject, the subject looks bright due to the light reflected by the subject. The AGC function closes the iris more and decreases light to darken the subject, thereby decreasing the luminance signal level of the whole screen. As a result, the environmental brightness is misdetermined as the dark state.